It is known to provide belt buckle devices for seat belts with drive devices for different purposes, using which the belt buckle can be extended or retracted. If the belt buckle in the set position is, for example, difficult to reach, the drive device can be provided for moving the belt buckle from the set position into an easier-to-reach presentation position. In addition, it is known for the belt buckle to abruptly retract from the set position into a taut position in a pre-accident phase or in an early phase of the accident in order to extract the existing belt slack in the seat belt of the seat belt device. This so-called pretensioning movement can be formed both irreversible using a pyrotechnic drive or preloaded spring, and reversible using an electric motor (for example) as a drive device.
In the case that the drive device is formed by an electric motor, this is designed as an electric motor having small physical size and having a very high rotational speed and a very low rotational inertia. The high rotational speed of the electric motor is useful since sufficiently high forces can be realized by the use of an appropriate transmission. The low inertia of the electric motor thus makes sense, since the high rotational speed of several thousand rotations per minute can thereby be achieved in a very short time of a few milliseconds. One possible embodiment of the transmission is comprised of providing a threaded sleeve on the draw cable of the belt buckle, which threaded sleeve is in engagement with a threaded spindle driven by the electric motor. Since the electric motor can also move the threaded sleeve in different directions by rotational movements in different directions, by using the electric motor as a drive device both the belt-buckle moving function and the tensioning function of the seat belt can be realized.
One problem to be solved is that the restraint force in an accident likewise must be transmitted into the vehicle structure via the threaded sleeve, the spindle, and finally the electric motor such that the shaft drive must be designed for forces that are greater by far than those that would be needed for the belt buckle feed function and the pretensioning function. It is particularly disadvantageous that the electric motor can also be subjected to stress, in particular in the axial direction of the shaft, such that the electric motor must be designed for forces of a magnitude that does not customarily arise during standard operation.
Against this background the object of the invention is to provide a belt buckle device for a seat belt including a drive device and a belt buckle that can be optimally designed for both the transmission of the drive movement of the drive device and for transmitting the restraining force in an accident.
A belt buckle device according to the invention including the features described herein is proposed for achieving the above mentioned object.